This invention relates to a mask support system for a color cathode ray tube having a flat faceplate, and to a system for indexing or referencing a flat tension shadow mask for insertion and removal during the panel screening process, and for repositioning the mask for welding to a permanent metal support frame.
A shadow mask or color selection electrode is a device which is disposed adjacent the luminescent phosphor screen that forms the target electrode of a color cathode ray tube, to control the landing pattern of one or more electron beams as they are swept across the screen. The shadow mask achieves color selection by partially shadowing the surface of the screen from scanning electron beams, permitting access to selected elemental phosphor areas by those beams. The most common type of color selection electrode used in color television receivers today is the conventional non-tensed curved type.
There is a tendency, however, for the conventional shadow mask utilized in color picture tubes to distort or buckle in the shape of a dome in those areas wherein concentrated beams of high brightness tend to heat the mask and cause localized distortion. The general practice of positioning the mask at an assigned location relative to the phosphor screen, by suspending it from three pre-selected points disposed about the periphery of the tube's face panel, accommodates overall thermal expansion of the mask, but does not resolve the above-described localized doming problem caused by concentrated heating in localized areas of the mask.
Tensioned shadow masks have been utilized with both the common cylindrical faceplate CRT and the newer flat faceplate CRT. With respect to the former, U.S. Pat. No. 3,638,063 discloses the use of a shadow mask in the form of a grid tensed across a spring frame suspended conventionally within the tube. The mask supporting frame is mechanically stressed, such as by compressing it prior to attaching the shadow mask thereto. Upon release of the compression force, restoration forces in the frame establish tension in the mask. Thus, the mask, while under tension, will not dome and retains its desired configuration during normal conditions. However, under extreme tube operating conditions, electron bombardment of the mask can cause a series of grids of the mask to relax and cause color impurities.
With respect to the latter, or flat faceplate construction, U.S. Pat. Nos. 4,547,696 and 4,593,224, set forth different forms of tension mask registration and support systems. The system disclosed in Pat. No. 4,547,696 requires the milling of precision cavities in both sealing surfaces of a glass mask support frame, in the sealing land of the faceplate, and in the sealing land of the funnel. In addition, a stabilizing or stiffening member must be frit sealed to the mask support frame in order to avoid any bending or flexing moment applied to the support frame by virtue of the tension forces in the mask. Finally, a plurality of spherical balls are positioned within the grooves between adjacent surfaces of the support frame and the faceplate to provide a registration and indexing means, usable when screening the pattern of elemental phosphor deposits upon the target surface of the panel. However, each time that the panel is removed from the support ring for applying the different phosphors, it would be imperative that each spherical ball be replaced in its original groove in order to avoid any disorientation which might be occasioned by variation in the ball sizes. The same requirement is true when finally frit sealing the support frame to the panel, wherein the frit material could jeopardize the Q-spacing previously relied upon in the lighthousing of the applied phosphor deposits.
The Pat. No. 4,593,224 discloses a rather complex electrode assembly including an upper glass frame member, a lower glass frame member, a temporary metal mount having a plurality of nuts welded thereto with threaded spindles extending therethrough, and V-grooves milled into the sealing lands of the glass faceplate and the funnel for receiving metal alloy bosses formed in the upper and lower frame members. A mask is drawn taughtly across the opening of the temporary metal mount and secured thereto by brazing or welding. A plurality of nuts are brazed or welded to the temporary metal mount, and a spindle is threaded through each such nut. The temporary metal mounting frame is then positioned over the lower glass frame member and the upper glass frame member is positined upon the lower member with the mask element held therebetween. The upper and lower frame members are then frit sealed to the shadow mask in an oven, and the mask is thermally tensioned in the process. A plurality of alignment V-grooves are milled into the sealing land of the glass faceplate, and a plurality of metal alloy bosses are formed in a lower sealing land of the lower frame member. Accordingly, the electrode assembly, without the temporary metal mount, is then utilized for depositing the elemental phosphors on the target area of the faceplate by aligning the metal alloy bosses with the V-grooves in the faceplate. Finally, the electrode assembly and the faceplate are frit sealed together utilizing the same alignment system, however, the Q-space could possibly be varied by the thickness of the frit between the electrode assembly and the faceplate from that utilized during the deposition of the phosphors. In addition, the mask is thermally distorted upon the second heating, required for sealing the electrode assembly to the faceplate, and upon cooling there is a possibility of some residual distortion.
An earlier example of a tensioned shadow mask for use in a color television CRT is described in U.S. Pat. No. 2,625,734. The tensed mask described therein was created by resort to a process called "hot-blocking". The practice was to insert a flat mask between a pair of frames which loosely received the mask. A series of tapped screws joining the two frames served to captivate the mask when the screws were subsequently drawn-down. The loosely assembled frame and mask was then subjected to a heat cycle by positioning heated platens adjacent the mask to heat and thereby expand it. The frame, however, was kept at room temperature. When the mask attained a desired expansion, the frame screws were tightened to captivate the mask in its expanded state. The heating platens were then removed. Upon cooling down to room temperature, the mask was maintained under tension by the frame. The resultant assembly was then mounted inside the tube adjacent the phosphor screen.
U.S. Pat. No. 2,813,213 describes a cathode ray tube which employs a switching grid mounted adjacent the phosphor screen. A taught wire grid is sealed in the tube envelope wall and an external frame is utilized to relieve the tension forces applied by the taught grid to the glass wall of the tube.
U.S. Pat. No. 3,284,655 is concerned with a direct viewing storage cathode ray tube employing a mesh storage target which is supported in a plane perpendicular to the axis of the tube. The mesh target comprises a storage surface capable of retaining a charge pattern which, in turn, controls the passage therethrough of a stream of electrons. From a structural standpoint, it is proposed that the mesh storage screen be affixed to a circumferential ring that is disposed across the open end of the envelope member. One end of the ring is in contact with the edge of the envelope member which has a coating of glass frit applied thereon. The end wall of another envelope member, also coated with frit, is placed in contact with the other side of the ring so that the end walls of the envelope members now abut both sides of the ring. Thereafter, this assembly is frit sealed to secure the ring and mesh target within the tube, however, the mesh screen is not said to be subject to tension forces, or is such screen a customary color selection electrode.
U.S. Pat. No. 3,894,321 is directed to a method for processing a color electrode ray tube having a thin foil mask sealed directly to the bulb. Included in this disclosure is a description of the sealing of a foil mask between the juncture of the skirt of the faceplate and the funnel. The foil mask is noted as having a greater thermal coefficient of expansion than that of the glass to which it is mounted, hence following a heating and cooling cycle in which the mask is cemented at the funnel-faceplate juncture, the greater shrinkage of the mask upon cooling places it under tension. The mask is shown to have two or more alignment holes near the corners of the mask which mate with alignment nipples in the faceplate. The nipples pass through the alignment holes to fit into recesses in the funnel. In another embodiment, the front panel is shown as having an inner ledge forming a continuous path around the tube, the top surface of which is a Q-distance away from the faceplate for receiving the foil mask such that the mask is sealed within the tube envelope. An embodiment is also shown in which the faceplate is skirtless and essentially flat.
U.S. Pat. No. 4,069,567 discloses a method of installing a shadow mask, such that under normal tube operating conditions, the mask is held by a holder in a hypertensive state, and is thus capable of withstanding an unusually high electron beam bombardment before relaxing. Preferably, the electrode is of a material which has a significantly higher coefficient of thermal expansion than that of its holder. The electrode and the holder may be externally heated together, such as by an oven, while the electrode is tensed. Simultaneously therewith selective heating is applied, such that the holder and electrode are caused to thermally expand, but the electrode by a greater amount. The electrode is affixed to the holder, and finally the electrode and holder are cooled to room temperature so as to hypertense the electrode due to the greater coefficient of thermal expansion and temperature fall of the electrode than that of the holder.
Finally, U.S. Pat. No. 4,595,857, which is similar to previously discussed Pat. No. 4,547,696, relates to a structure wherein the mask is sized and the frame is so adapted such that the mask is supported completely within the tube enclosure on a peripheral frame surface facing away from the faceplate.
In order to overcome the problems and complexities with the above-noted tension mask systems of the prior art, it is an object of the present invention to provide an accurate and simple positioning system for a tension mask in a CRT by minimizing the number of parts required and reducing costly machining and process steps.